Conventionally, when an overcurrent that flows in an electric circuit is detected, a current is detected by a detecting circuit. With the detected current, it is determined whether or not the detected current is an overcurrent. For example, as shown in FIG. 1, a current is detected by a detecting circuit 151. The detected current is supplied to a controlling circuit 152. The controlling circuit 152 determines whether or not the supplied current is an overcurrent. The determined result is supplied as a control signal to a driving circuit 153.
The driving circuit 153 magnetizes or demagnetizes a solenoid type switch 154 in accordance with the supplied control-signal. For example, when the solenoid type switch 154 is magnetized, it is turned off. In contrast, when the solenoid type switch 154 is demagnetized, it is turned on.
Conventionally, an electric circuit that uses a solenoid type switch is controlled by one driving coil (refer to Japanese Patent Laid-Open Publication No. HEI 05-290707).
Japanese Patent Laid-Open Publication No. 2002-093466 describes a technology of a charging controlling circuit that has a battery protecting function so as to integrate circuit boards for a charging controlling circuit and a battery cell protecting circuit as one board and allow a battery pack to be composed of only battery cells.
However, in such a circuit, if the detecting circuit and the controlling circuit are broken with an overcurrent, the possibility of which the solenoid type switch is kept on is high.
However, there is no method for determining whether or not the solenoid type switch 154 has been turned off. Thus, if the solenoid type switch 154 breaks and melt-adheres, it cannot be turned off.
To solve such a problem, another technology as shown in FIG. 2 has been disclosed. In the technology, a protecting circuit is disposed along with a switch 1121 as a solenoid type switch. The switch 1121 as a solenoid type switch is disposed between a positive terminal of a secondary battery cell 1131 and a protecting circuit 1134. Detecting circuits 1132 and 1133 are disposed in parallel with the secondary battery cell 1131. The detecting circuits 1132 and 1133 detect for example voltages or currents. A detected result of the detecting circuit 1132 is supplied to a driving circuit 1123. The driving circuit 1123 controls on/off states of the solenoid type switch 1121 in accordance with the detected result. Likewise, a detected result of the detecting circuit 1133 is supplied to the protecting circuit 1134. The protecting circuit 1134 shuts off for example a charging path and/or a discharging path so as to protect the secondary battery cell 1131 in accordance with the detected result of the detecting circuit 1133.
Thus, conventional technologies require an extra protecting circuit that protects a solenoid type switch.
Therefore, an object of the present invention is to provide a switching circuit, a switching method, and a protecting device that are capable of preventing a contact of a switch from adhering without need to dispose an extra detecting circuit, detecting on/off states of the switch, and maintaining safety without need to dispose an extra protecting circuit.
In addition, in a battery pack having a plurality of secondary battery cells connected in series, they are charged or discharged at a time. For example, a battery pack shown in FIG. 3 uses a battery block 2081 of which four secondary battery cells are connected in series and in parallel. The secondary battery cells are charged and discharged through terminals 2083 and 2084. As a protecting circuit that maintains safety of the battery block 2081, a switching circuit 2082 is disposed between the battery block 2081 and the terminal 2082.
Japanese Patent Laid-Open Publication No. HEI 7-322512 describes a battery assembly of which electrodes of a plurality of battery cells are not contacted to each other. In the battery assembly, the capacity of each battery cell is detected. Each battery cell is independently charged.
In a battery pack shown in FIG. 4, many battery cells are connected in a battery block 2088. In the battery pack, if for example one battery cell is short-circuited, an overcurrent flows in the battery cell. As a result, safety of the battery pack cannot be maintained.
Thus, to assure safety of a battery pack, the number of secondary battery cells that can be connected in series and in parallel has been restricted. Thus, the upper limit of the capacity of a battery pack has been designated.
Therefore, an object of the present invention is to provide a battery pack having at least two battery pack cells whose safety is assured and allowing them to be selectably used.